Stability of arc with DC

[spaf4]

I was brought up on AC machines, but occasionally used DC which, to my mind, were awful. The arc was never stable and wandered around all over the place. Swapping polarity and wrapping the cable around the job made no difference.

So what I am wondering now is, have things improved with the latest inverters. I know DC is easy to strike up and runs more smoothly (supposedly), but that's no use if the tip of the rod burns down erratically requiring continual adjustment to the rod angle.

 

[shenion]

I have had near no issues on my transformer arc on DC. It is 90OCV so arc is always stable. I have rarely seen arc blow.

Inverters make the current always constant and can have hot-start and anti-stick features. There will be little or no current slope (droop) like on transformer welders.

 

[gerry3]

"Arc Blow" using DC current is one of the main problems.
The cause is that a magnetic field is created when ever a current flows along a wire, with AC current the opposing half cycle cancels this out.
Most times this occurs when welding comes near the end of a butt weld or when approching an inside corner.
In the case of the butt weld, a tack, at the ends of the joint will normally cure this.
The inside corner joint is more difficult to correct.
Sometimes winding the earth cable round the job will help but I find that if you start in the corner and work out wards this will help.
I have found that modern welding sets are not prone to this as DC generators are.
My TIG welder when used for DC Arc welding does not have arc blow, so it must be a case of how the welding current is produced by the set and at what the OCV is.

 

[SiPMerlin150]

I was brought up on AC machines, but occasionally used DC which, to my mind, were awful. The arc was never stable and wandered around all over the place. Swapping polarity and wrapping the cable around the job made no difference.

So what I am wondering now is, have things improved with the latest inverters. I know DC is easy to strike up and runs more smoothly (supposedly), but that's no use if the tip of the rod burns down erratically requiring continual adjustment to the rod angle.

I was just about to start a thread like this, but you have nailed it on the head, this is what i'm experiencing, only that the time where I use DC is at my college, and those machines are the larger cebora water cooled TIG/MMA process machines. The arc likes to move around! I prefer my AC too!

 

[spaf4]

Now, no disrespect to anyone in any way shape or form as I appreciate we may well all have different abilities. But hot start and anti-stick features are of no use to me if you can't put down a uniform weld in any given position time after time after time. As for starting in corners and working outwards, well, I would literally beat any apprentice I was teaching if I caught them doing that.

In a lot of situations if you've a corner filet to weld round you'll have another previously welded fillet running vertically starting in the same corner. Now, the best way to tidy up the beginning of the vertical fillet so that it's pleasing on the eye, is to run your weld in and around the corner to seal the end.

We welders are notorious for looking at the quality of other welders work. So if one sees that you have corner welds to carry out, I'll guarantee at some point he'll take a look to see how you've done them. For those new to this game the best way of learning to weld round corners is to tack bits of angle iron to a flat plate and practice.

Now, where can I get a compact AC machine.

 

[matt1978]

I may be wrong but I don't think a ACDC MMA only inverter is available on the market. If you want ACDC for MMA and it has to be an inverter then you would need to look for a ACDC Tig.

You can still get fairly compact (but heavy) Transformer buzz boxes.

 

[matt1978]

Also apart from when magnetic issues have been the cause I have never any issues with the stability of the arc on DC. It's only ever so occasionally on welding into corners or if there has been a large magnetic build up but usually changing welding direction and/or moving the earth usually solves it.

Most (in fact i cant think of any that dont) MMA inverters will have a hot start, anti stick and maybe even a variable arc force built in

 

[PTvor]

Now, where can I get a compact AC machine.

Compact or compact and portable?

The problem with an AC machine is the mains transformer, which is bound to be big and heavy especially if it has to have a good duty cycle and long life. You're then stuck with oil cooling or a big machine with air cooling.

The other way of doing it is as in AC TIG machines where there's a second inverter stage (with all the expense and complexity) to turn DC back to AC.

For the money it takes to make a good quality transformer based machine, with a good duty cycle, you may as well make make an inverter, which most people prefer and is where the market has moved.


I believe that buzz boxes made 20 or 30 years ago were of better quality, because they were the only portable arc welders available and they were taken seriously. They were more than the bargain basement option which can weld a bit. Copper was cheaper back then too.

 

[spaf4]

Well, seeing how I can't find an oil filled Oxford with a shoulder strap , I've ordered a Parweld XTi 160 instead. As soon as I've put it through it's paces I'll let you know how it behaved itself and perhaps include a few pics of the resultant welds. This should be interesting as I haven't burnt a rod in over 10 years.

 

[spaf4]

Talked to a good friend last night. Excellent welder and moved on to become an aircraft welding inspector. Got onto the topic of Arc Blow and he told me that connecting another cable to your existing earth cable plug, to give you two earth leads, helps greatly to minimise problems with the arc wandering about.

Never heard of this one before, but you live and learn I guess and I may just give this a go out of interest.

 

[SiPMerlin150]

Arc blow sucks... Its one of the reasons why I prefer AC to DC, but prefer DC because all rods except for AC designated rods burn happily on DC ....enough said !

 

[wheresmejumper]

i normally earth the bench(inverter) but if the arc starts to wander,earthing the workpiece helps

 

[SiPMerlin150]

Its weird, and it screws up my runs, especially on tee fillets, I've found that if the work return clamp is on one side of the bench theres a tendency for the arc to pull more towards the direction this clamp is situated. Magnetic holding clamps to position metal pieces are good for disrupting the arc also.

This is why I love AC so much, I'll run it where I can .

 

[spaf4]

I'm with you SiPMerlin. AC is always stable.

That said the Parweld I've just started using is behaving itself at the moment. I managed to run a three bead filet towards and around an inside corner without a problem. I find that if the arc is going to wander, it is often just as you start a run. Run on and run off plates are helpful in this respect.